Equation 24.2 where DLCO = diffusion capacity of the lung for carbon monoxide.
Assessment of DLCO has become a classic measurement of the diffusion barrier of the alveolar-capillary membrane. It is useful in the differential diagnosis of certain obstructive lung diseases, such as emphysema.
Apatientwithinterstitialpulmonaryfibrosis(arestrictivelungdisease)inhalesasinglebreathof0.3%COfromresidualvolumetototallungcapacity.Heholdshisbreathfor10secondsandthenexhales.Afterdiscardingtheexhaledgasfromthedeadspace,arepresentativesampleofalveolargasfromlateinexhalationiscollected.TheaveragealveolarCOpressureis0.1mmHg,and0.25mLofCOhasbeentakenup.ThediffusioncapacityforCOinthispatientis . mL/10 seconds × 0 1. mm Hg = 15 mL/minute/mm Hg
ThenormalrangeforDLCOis20to30mL/minute/mmHg.Patientswithinterstitialpulmonaryfibrosishaveaninitialalveolarinflammatoryresponsewithsubsequentscarformationwithintheinterstitialspace.Theinflammationandscarreplacethealveolianddecreasethesurfaceareaforgasdiffusiontooccur,whichresultsindecreasedDLCO.Thisisaclassiccharacteristicofcertaintypesofrestrictivelungdisease.
Oxygen and Carbon Dioxide Exchange in the Lung Is Perfusion Limited
Different gases have different solubility factors. Gases that are insoluble in blood (i.e., anesthetic gases such as nitrous oxide and ether) do not chemically combine with proteins in blood and equilibrate rapidly between alveolar gas and blood. The equilibration occurs in less time than the 0.75 seconds that the red blood cell spends in the capillary bed (capillary transit time). The diffusion of insoluble gases between alveolar gas and blood is considered perfusion limited because the partial pressure of gas in the blood leaving the capillary has reached equilibrium with alveolar gas and is limited only by the amount of blood perfusing the alveolus. In contrast, a gas that is diffusion limited, such as CO, has low solubility in the alveolar-capillary membrane but high solubility in blood because of its high affinity for hemoglobin (Hgb). These features prevent the equilibration of CO between alveolar gas and blood during the red blood cell transit time.
The high affinity of CO for Hgb enables large amounts of CO to be taken up in blood with little or no appreciable increase in its partial pressure. Gases that are chemically bound to Hgb do not exert a partial pressure in blood. Like CO, both CO2 and O2 have relatively low solubility in the alveolar-capillary membrane but high solubility in blood because of their ability to bind to Hgb. However, their rate of equilibration is sufficiently rapid for complete equilibration to occur during the transit time of the red blood cell within the capillary. Equilibration for O2 and CO2 usually occurs within 0.25 seconds. Thus O2 and CO2 transfer is normally perfusion limited. The partial pressure of a gas that is diffusion limited (i.e., CO) does not reach equilibrium with the alveolar pressure over the time that it spends in the capillary (
Fig. 24.3 ). Although CO2 has a greater rate of diffusion in blood than O2 does, it has a lower membrane-blood solubility ratio and consequently takes approximately the same amount of time to reach equilibration in blood.
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